Radiography belongs to inspection means necessary for customs, airports, and railway systems. With a principle of interactions occurring by X-rays penetrating an object, radiography enables content inside a box to be imaged without opening the box, which can effectively identify contrabands such as guns, explosives, drugs in luggage, and have positive effects on protecting citizen's personal and property safety and maintaining social stability.
In the current radiography technology, a main approach of non-destructive testing (NDT) is transmission imaging technology, which utilizes a principle in which X-rays penetrating substance with different densities and materials will generate different attenuations, for implementing non-open-package inspection on freight container, luggage articles. For example, in a patent application with a publication number CN102804326A, the applicant proposed a radiography security inspection system using the X-ray transmission principle, which is mainly constituted by a ray source arranged at one side of an object to be inspected for generating X-rays, a detector module arranged at the other side of the object to be inspected for receiving the ray source, an inspection area for placing the object to be inspected, a data processing unit and a man-machine interaction unit. The above patent also discloses a system and a method for multi-view transmission imaging, which uses a plurality of ray sources and detector modules to form a plurality of scan planes. Respective scan planes scan independently to obtain perspective images of the target in multiple angles, so as to avoid problems of overlapping and difficult recognition of the objects in a single angle view. In addition, dual-energy and multi-energy imaging are also widely used in the transmission imaging technology. For example, a patent application with a publication number CN 102484935A discloses a multi-energy transmission imaging system, which implements energy spectrum decomposition by multiple layers of detector modules corresponding to different energies and by utilizing difference in attenuation capabilities of X-rays with different energies in penetrating substance, so as to obtain estimation on substance atomic number, electron density etc. of the perspective image.
Regarding the security inspection technology, how to identify contrabands in target goods better is core content thereof. Conventional perspective imaging technology improves contraband identification performance mainly by increasing the scan view angle and using dual-energy/multi-energy scan, which has problems as follows.
On one hand, a method of increasing the number of pairs of ray sources and detectors implements perspective scan in different view angles, facilitating to solve the problems of overlapping and difficult recognition for perspective imaging in the single view angle. However, using the scheme of the plurality of ray sources and detectors will considerably increase overall system cost. Moreover, since different pairs of ray sources-detectors need to operate independently, a certain distance interval needs to be maintained between respective scan planes. Therefore, a dual-view-angle/multi-view-angle perspective system always covers a larger area, which limits flexibility and application scope of the system to some extent.
On the other hand, the method of dual-energy/multi-energy imaging may be used for implementing calculation of physical parameters such as atomic number, electron density of substance, facilitating to improve identification capability of contrabands. However, the dual-energy/multi-energy imaging generally uses multiple layers of detectors for implementing differentiated collections of transmitted rays with different energies, which means that the number of detector crystal units and the number of read-out circuits are required to be increased. Considering cost of the detector unit is expensive, using multiple layer detectors will increase the overall system cost, either.